1. Field of the Invention
The preset invention relates to an optical apparatus including non-linear optical crystal, and a wavelength conversion laser light source, an image display apparatus and a laser light source device adopting such optical apparatus.
2. Description of the Background Art
Conventionally, a variety of wavelength conversion laser light sources have been developed and made into practical applications, wherein a visible laser beam is obtained, such as a green light or an ultraviolet ray which is obtained by further converting the green light, etc., through the wavelength conversion using the non-linear optical effects of a laser beam emitted from the Nd: YAG laser, or the Nd: YVO4 laser, for example, as disclosed in Japanese unexamined Patent Publication No. 2004-157217 and Japanese unexamined Patent Publication No. 2000-305120. These converted light beams are used for laser processing, or a laser display, etc.
FIG. 1 shows a typical structure of a conventional wavelength conversion laser light source using the non-linear optical effect. In order to obtain the non-linear optical effect, it is required to adopt the non-linear optical crystals having the birefringence. Examples of such non-linear optical crystals having the birefringence include: LiB3O5 (LBO: lithium triborate), KTiOPO4 (KTP: Potassium Titanyl Phosphate), CsLiB6O10(CLBO: Cesium Lithium Borate); or LiNbO3 (PPLN:Lithium Tantalate), and LiTaO3(PPLT: Lithium Tantalate) having a periodical polarization inversion structure, etc.
As shown in FIG. 1, a wavelength conversion laser light source 100 includes a fundamental wave light source 101, a collective lens 108, a non-linear optical crystals (wavelength conversion element) 109, a re-collimating lens 111, a wavelength-dividing mirror, a temperature holder 116 such as a heater or the like for holding the temperature of the non-linear optical crystals constant, a control unit 115 for controlling a laser output, and a temperature controller 122 for controlling the temperature of the non-linear optical crystals provided in the control unit 115. For the fundamental wave light source 101, Nd: YAG laser, Nd: YVO4 laser, fiber laser using Yb doped fiber having a wavelength of 1.06 μm are generally used.
Here, the actual operations will be explained, which generate the second harmonic wave having a wavelength of 0.532 μm which is around ½ of wavelength (1.06 μm) of the fundamental wave.
The laser beam having a wavelength of 1.06 μm as emitted from the fundamental wave light source 101 is converged into the non-linear optical crystals 109 by the collective lens 108. Here, the non-linear optical crystals 109 needs to have the refractive index for the light having the wavelength of 1.06 μm matched with the refractive index for the light having the wavelength of 0.532 μm to be generated (phase matching condition). Generally, the refractive index for the crystals varies according to temperature conditions of the crystals. Therefore, the temperature of the crystals needs to be maintained constant. For this reason, the non-linear optical crystals are placed in the temperature holder 116, and are maintained at a predetermined temperature suited for the kind of the crystals. For example, when adopting the LBO crystals, in order to obtain the type-1 non-critical phase matching (the phase matching state), the LBO crystals need to be maintained at a temperature in a range of 148° C. to 150° C.
On the other hand, when adopting LiNbO3 crystals having a periodical polarization inversion structure, it is possible to determine the temperature and the wavelength for the phase matching condition by selecting the period for the periodical polarization inversion structure. However, in order to maintain the phase matching condition, it is required to keep the element temperature of the wavelength of the fundamental wave constant (see Japanese unexamined Patent Publication No. 2004-157217 and Japanese unexamined Patent Publication No. 2000-305120). Incidentally, the method of obtaining a phase matching condition by adjusting the temperature using LiNbO3 crystals without having a periodical polarization inversion structure is disclosed, for example, Japanese Patent No. 3412906.
However, it has been found that for some kinds of the non-linear optical crystals to be adopted as the wavelength conversion element, the temperature of the element is raised by absorbing the fundamental wave and the harmonic wave as generated, which makes the phase matching temperature (wavelength) vary according to the output level of the harmonic wave, thereby presenting a problem in that a high conversion efficiency cannot be realized.